This invention relates to methylene-bis-(cyclohexyl isocyanates) which are mono-substituted on only one cyclohexane ring. The present invention also relates to a process for the preparation of such isocyanates.
Aliphatic and cycloaliphatic diisocyanates, such as hexamethylenediisocyanate, 3,5,5-trimethyl-1-isocyanato-3-isocyanatomethyl-cyclohexane and 4,4'-methylene-bis-(cyclohexylisocyanate) and mixtures of their position isomers and/or stereoisomers are used in polyurethane chemistry. For example, such diisocyanates may be used in the production of lightfast coating materials with good weather resistance. They may also be readily worked up into lacquer binders, elastomers or foams by a reaction with polyols if the diisocyanate is liquid at room temperature and compatible or sufficiently miscible with the polyols (see, e.g. German Offenlegungsschrift No. 1,768,832). Pure trans,trans-4,4'-methylene-bis-(cyclohexyl isocyanate), for example, is not suitable for this purpose because it is solid at room temperature (melting point 83.degree. C.) and only slightly soluble in polyols. It is therefore removed from the reaction mixture by crystallization before the polyaddition reaction is completed. The stereoisomeric mixture of trans,trans-, cis,trans- and cis,cis-4,4'-methylene-bis-(cyclohexylisocyanate) (which may be obtained by hydrogenation of 4,4'-diaminodiphenylmethane on the nucleus followed by phosgenation) is also solid at room temperature since the trans,trans-diamine is the main product obtained when hydrogenation is carried out under normal conditions. Its usefulness for the production of the above-mentioned polyurethanes is therefore limited.
Although removal of the trans,trans-4,4'-methylene-bis-(cyclohexylisocyanate) from the stereoisomeric mixture is technically possible (e.g., by precipitation of the trans,trans-isomers in the form of carbamic acid chloride followed by filtration (see Japanese Patent Publication No. 53 046-944 of Apr. 27, 1978, this additional process step results in a reduction in isocyanate yield, which must be regarded as a serious disadvantage.
Liquid cycloaliphatic diisocyanates may be prepared according to the prior art by phosgenation of 2,4'-methylene-bis-(cyclohexylisocyanate) (see German Offenlegungsschrift No. 1,768,832). According to this Offenlegungsschrift, mixtures of 2,4'- and 4,4'-isomers are liquid even when the 2,4'-isomer content amounts to 30 to 95 wt % and the 4,4'-isomer has a trans-trans-isomer content of less than 50 wt %. It was found to be a disadvantage, however, that the 2,4'-isomer is difficult to obtain in the pure form since it is prepared in three stages (i.e., the condensation of aniline with formaldehyde, hydrogenation on the nucleus and phosgenation) and the 2,4'-diaminodiphenylmethane obtained at the stage of condensation only amounts to about 30% of the theoretical yield even under optimum conditions (see German Offenlegungsschrift No. 1,937,685). Consequently, a technically elaborate process of separation of isomers would be required to isolate the pure 2,4'-isomer.
The direct preparation of a methylene-bis-(cyclohexylisocyanate) rich in 2,4'-isomer by the phosgenation of a corresponding mixture of methylene-bis-(cyclohexylamine) isomers also has its disadvantages because the 2,4'-diaminodiphenylmethane content at the stage of aniline/formaldehyde condensation correlates with that of the 2,2'-isomer which is subject to a decomposition reaction at the stage of hydrogenation at the nucleus.